What is the difference between CloseableHttpClient

问题:

I'm studying an application developed by our company. It uses the Apache HttpClient library. In the source code it uses the HttpClient class to create instances to connect to a server.

I want to learn about Apache HttpClient and I've gone trough this set of examples. All the examples use CloseableHttpClient instead of HttpClient. So I think CloseableHttpClient is an extended version of HttpClient. If this is the case I have two questions:

• What is the difference between these two?
• Which class is recommended to use for my new development?

回答1:

• The main entry point of the HttpClient API is the HttpClient interface.
• The most essential function of HttpClient is to execute HTTP methods.
• Execution of an HTTP method involves one or several HTTP request / HTTP response exchanges, usually handled internally by HttpClient.

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• CloseableHttpClient is an abstract class which is the base implementation of HttpClient that also implements java.io.Closeable.

• Here is an example of request execution process in its simplest form:

  CloseableHttpClient httpclient = HttpClients.createDefault();
  HttpGet httpget = new HttpGet("http://localhost/");
  CloseableHttpResponse response = httpclient.execute(httpget);
  try {
      //do something
  } finally {
      response.close();
  }

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• HttpClient resource deallocation: When an instance CloseableHttpClient is no longer needed and is about to go out of scope the connection manager associated with it must be shut down by calling the CloseableHttpClient#close() method.

  CloseableHttpClient httpclient = HttpClients.createDefault();
  try {
      //do something
  } finally {
      httpclient.close();
  }

see the Reference to learn fundamentals.

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@Scadge Since Java 7, Use of try-with-resources statement ensures that each resource is closed at the end of the statement.

try(CloseableHttpClient httpclient = HttpClients.createDefault()){
//do something with httpclient here
}

回答2:

The other answers don't seem to address why close() is really necessary? * 2

Doubt on the answer "HttpClient resource deallocation".

It is mentioned in old 3.x httpcomponents doc, which is long back and has a lot difference from 4.x HC. Besides the explanation is so brief that doesn't say what this underlying resource is.

I did some research on 4.5.2 release source code, found the implementations of CloseableHttpClient:close() basically only closes its connection manager.

(FYI) That's why when you use a shared PoolingClientConnectionManager and call client close(), exception java.lang.IllegalStateException: Connection pool shut down will occur. To avoid, setConnectionManagerShared works.

I prefer not do CloseableHttpClient:close() after every single request

I used to create a new http client instance when doing request and finally close it. In this case, it'd better not to call close(). Since, if connection manager doesn't have "shared" flag, it'll be shutdown, which is too expensive for a single request.

In fact, I also found in library clj-http, a Clojure wrapper over Apache HC 4.5, doesn't call close() at all. See func request in file core.clj

回答3:

Had the same question. The other answers don't seem to address why close() is really necessary? Also, Op seemed to be struggling to figure out the preferred way to work with HttpClient, et al.

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According to Apache:

// The underlying HTTP connection is still held by the response object
// to allow the response content to be streamed directly from the network socket.
// In order to ensure correct deallocation of system resources
// the user MUST call CloseableHttpResponse#close() from a finally clause.

In addition, the relationships go as follows:

HttpClient (interface)

implemented by:

CloseableHttpClient - ThreadSafe.

DefaultHttpClient - ThreadSafe BUT deprecated, use HttpClientBuilder instead.

HttpClientBuilder - NOT ThreadSafe, BUT creates ThreadSafe CloseableHttpClient.

• Use to create CUSTOM CloseableHttpClient.

HttpClients - NOT ThreadSafe, BUT creates ThreadSafe CloseableHttpClient.

• Use to create DEFAULT or MINIMAL CloseableHttpClient.

The preferred way according to Apache:

CloseableHttpClient httpclient = HttpClients.createDefault();

The example they give does httpclient.close() in the finally clause, and also makes use of ResponseHandler as well.

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As an alternative, the way mkyong does it is a bit interesting, as well:

HttpClient client = HttpClientBuilder.create().build();

He doesn't show a client.close() call but I would think it is necessary, since client is still an instance of CloseableHttpClient.

回答4:

In the next major version of the library HttpClient interface is going to extend Closeable. Until then it is recommended to use CloseableHttpClient if compatibility with earlier 4.x versions (4.0, 4.1 and 4.2) is not required.

回答5:

HttpClient is not a class, it is an interface. You cannot use it for development in the way you mean.

What you want is a class that implements the HttpClient interface, and that is CloseableHttpClient.

回答6:

CloseableHttpClient is the base class of the httpclient library, the one all implementations use. Other subclasses are for the most part deprecated.

The HttpClient is an interface for this class and other classes.

You should then use the CloseableHttpClient in your code, and create it using the HttpClientBuilder. If you need to wrap the client to add specific behaviour you should use request and response interceptors instead of wrapping with the HttpClient.

This answer was given in the context of httpclient-4.3.